Current driver



y 1969 N. M.SHUKLA 3,446,984

' CURRENT DRIVER Filed July 16, 1965 Fig. 1

44 48 m off 5 F time curr nt INVENTOR. NARENDRA M. SHUKLA ATTORNEY N. M.SHUKLA 3,446,984.

C URRENT DRIVER May 27, 1969- Filed July 16, 1965 Fig.4 T 88 g 96 I 98 gi 100 U L. I s l time IN V EN TOR. NARENDRA M. SHUKLA ATTORNEY UnitedStates Patent ABSTRACT OF THE DISCLOSURE A current driver circuit forproviding a high energy current pulse to the set windings of a pluralityof ferromagnetic cores. The high energy current pulse is due to thedischarge of the large inductance of an inductor through the smallerinductance of the set windings. A low power transistor interrupts theflow of charging current through the inductor and gating means, having apredetermined voltage threshold, electrically connects the set windingsof the ferromagnetic cores to the inductor thereby providing thedischarge path for said inductor. The transistor additionally controls aseparate low-current path through the reset windings of theferromagnetic cores maintaining the cores in a reset condition duringthe charging time of the inductor.

This invention relates to apparatus for providing electrical power toutility devices, and more particularly, to pulse drivers for providingintermittent pulses of power to an electrical device.

It is frequently necessary to activate electrical units with power inthe form of a series of pulses. For example, a device requiring thistype of power is the keyboard transducer disclosed in the application ofFlavius A. Mathamel, Ser. No. 433,359, filed Feb. 17, 1965, and assignedto the same assignee as this application. This transducer requires thata series of pulses be applied to a ferromagnetic core to switch thiscore from one remanent state to the opposite remanent state. It isintended that a plurality of these transducers have their corescontinuously pulsed by a switching current.

The cores are normally inhibited from switching. However, when aselected key is depressed on a keyboard, some of the cores are releasedand provide output pulses from their output windings to a dataprocessing device. The outputs of the keyboard transducers indicatewhich key has been depressed by a coded plurality of output signals.

It is desirable that the current drivers used to provide the pulsedenergy to the cores be simple and economical. Because of this,semi-conductor devices are frequently used in the current drivercircuitry. However, when magnetic cores are switched, they present avarying impedance to the driver. This characteristic is frequentlyencountered in devices which are activated by a series of energy pulses.The drivers, to operate efiiciently, must have a high output impedance.However, if resistors are used to increase the output impedance of asemi-conductor or if a power transistor connected in the common-basemode is used, a great deal of power is dissipated. This, of course, isundesirable.

Also, it is frequently necessary to provide AC pulses. However, ACgenerators are expensive. If two DC pulse generators having oppositepolarity outputs are used, there is a duplication of equipment.Accordingly, it is an object of this invention to provide improvedapparatus for applying energy pulses to a utility device.

It is a further object of this invention to provide a current pulsedriver which is simple, economical, and which does not dissipateexcessive amounts of power.

Patented May 27, 1969 It is a further object of this invention toprovide a pulse generator which is capable of both setting and resttingferromagnetic cores without excessive duplication of components andwhich automatically resets the cores if they are set by noise.

In accordance with the above objects, a current driver is providedhaving a source of voltage, an inductor, gating means such as a diode,and a switch. The diode has a higher forward impedance than the switchwhen the switch is closed. One end of the inductor is connected to thevoltage source and the other end is connected to both the diode and theswitch. The windings of the cores are connected in series with the diodeand are grounded at the other end so that current may flow from thepower supply, through the inductor, the diode, and all of the windings,to ground. The switch short circuits the diode to ground while it isclosed and presents a high impedance to ground while it is open.

The switch is normally closed so that no current flows through thewindings of the cores. However, when it is desired to set the cores, theswitch is opened. When the switch is opened the energy stored in theinductor discharges to ground through the diode and through the windingsof the cores, setting each of the cores. It is noted that the switch maybe a relatively inexpensive transistor connected in the common-emitterconfiguration. Therefore it is possible to fabricate a simple,economical, driver which does not dissipate large amounts of power. Asimilar driver may be used toreset the cores by connecting its output toreset windings wound in the opposite direction as the set windings.

It is possible'to use the driver of this invention'to both set and resetthe cores. This is accomplished by connecting one end of the series ofreset windings of the cores to the power supply and connecting the otherend of the series-connected reset windings to the point in the circuitwhich is electrically between the diode and the switch. The resistancein this reset circuit is adjusted so that current fiows through itwhenever the switch is closed. This current resets the cores. It isnoted that current at the same time must flow through the alternate pathbetween the power supply and ground, which path includes the inductor.

The above noted and other features of the invention will be understoodmore clearly and fully from the following detailed description whenconsidered with reference to the accompanying drawings in which:

FIG. 1 is a schematic circuit diagram of a current driver according toan embodiment of the invention;

FIG. 2 is a graph of certain wave forms which occur during the operationof the circuit of FIG. 1;

FIG. 3 is a schematic circuit diagram of an embodiment of the inventionin which a current driver is capable of applying pulses to a pluralityof diifreent terminals of a load;

FIG. 4 is a graph of certain wave forms which occur during the operationof the circuit of FIG. 3; and

FIG. 5 is a schematic circuit diagram of another embodiment of theinvention.

In FIG. 1 a schematic circuit diagram of a current driver is shownhaving an input terminal .10 for receiving trigger pulses and having anoutput terminal 12 for providing output current pulses to the setwindings of a plurality of cores 1.4, which windings are connected inseries with each other between the output terminal 12 and ground. Afirst NPN transistor 16 has its base electrically connected to the inputterminal 10 through a resistor 18 and to a source of negative potential20 through the resistor 22. Its emitter is grounded, and its collectoris electrically connected to a source of positive potential 24 through aresistor 26.

A second NPN transistor 28 has its base electrically connected to thecollector of the transistor 16 through the resistor 30 and to the sourceof negative potential 20 through a resistor 32. 'Its emitter isgrounded, and its collector is electrically connected to the source ofpositive potential 24 through an inductor 34 and to the gating meanssuch as the anode of the diode 36-. The cathode of the diode 36 iselectrically connected to the output termirial 12.

The transistor 16 is biased to be normally nonconducting; the transistor28 is biased to be normally conducting. When a positive trigger pulse isapplied to the input terminal 10, the transistor 16 is driven tosaturation causing the potential on its collector to become lesspositive. The base of the transistor 28 becomes negative since it isconnected to the collector of the transistor .16 causing the transistor28 to be cut off. Now the current from the source 24 which has beenflowing through the inductor 34 to ground through the transistor 28 isforced to find an alternate path to ground. This path is through thediode 36 and the set windings of the cores 1 4.

The inductor 34 has a much larger inductance than the set windings ofthe cores 14. The energy stored in the inductor 34 sends a largetransient current through the windings of the cores 14 whenever itsnormal current fiOlW is interrupted by opening the transistor switch 28.The current flowing through the set windings while the transistor switch28 is closed is not large enough to set the cores 14 because the forwardresistance of the diode 36 is greater than the collector-to-emitterresistance of the transistor 28. The voltage swing on the collector isequal to the back voltage or induced voltage in the primary windings ofthe setting cores. The current through the switching core isapproximately constant because of the energy stored in the inductorduring the time when the transistor switch was closed.

The transistor 28 may be a medium-powered transistor. If a transistorwere to be used to directly drive the cores 14 without the benefit ofstored energy in an inductor, it would have to be a high-polwer fasttransistor. Also, in such a case, some provision would have to be madefor increasing the output impedance of the transistor to provideefficient power transfer to the cores 14 when the input impedanceincreases during switching.

In FIG. 2, a graph of three curves is shown ha'ving common abscissae oftime and individual ordinates of current. The curve 38 represents thecurrent flow through the transistor 28. This current is normally largeas indicated by the portion of the curve 40. However, when a positivetrigger pulse is applied to the terminal 10, the current through thetransistor 28 ceases to flow as indicated at 42 on the curve 38.

The curve 44 represents the current flow through the set windings of thecores 14. When the current 40 is flowing through the transistor 28, nocurrent flows through the cores as indicated by the region 46 on thecurve 44. However, when a positive trigger pulse is applied to the inputterminal cutting olf the transistor 28, a high current 48 flows throughthe set windings of the cores 14. The curve 50 represents the currentthrough the inductor 34. It is seen that this current is continuous andmay be thought of as the sum of the currents represented by the curves38 and 4 4. It is the resistance to a change in current of the inductor34 that provides the eflicient switching of the cores 14.

In FIG. 3, a schematic circuit diagram of a modification of theinvention is shown having a set line 52 capable of setting thirteencores 54 in response to a 4-microsecond, 6 volt input pulse applied tothe input terminal 56 and having a reset line 58 capable of resettingthe thirteen :cores 54 during the 64-microsecond interval between theinput pulses applied to terminal 56. The set windings of the cores 54are connected in series with each other. One end of the series ofwindings is grounded and the other end is connected to the set line 52.Also, the reset wind- 4 ings of the cores 54 are connected in serieswith each other. One end of this series connection is connected to thereset line 58 and the other end is connected to the collector of a2N2102, NPN transistor 60. Each of the thirteen cores may be used in aseparate transducer for the keyboard described in the above-identifiedpatent application to Flavius A. Mathamel.

The base of a 2N708, NPN transistor 62 is electrically connected to theinput terminal 56 through a 470 ohm, A watt resistor 64 and to a source66 of a negative 8 volts through the 6.8K (Kilo-ohm) /4 watt resistor68. The emitter of the transistor 62 is grounded and its collector iselectrically connected to a source 70 of a positive 24 volts through a360 ohm, 1 watt resistor 72.

This stage of the current driver serves primarily as an inverter toinvert the positive pulses applied to the input terminal 56 to negativepulses at the next stage of the current driver. The transistor 62 isnormally cut off. However, the 6 volt, 4 microsecond input pulsesapplied to the terminal 56 bias it to saturation for a short time. Whenit is biased to saturation, the voltage on its collector drops from apotential of approximately 4.5 volts received from the source 70 to apotential close to ground received from the emitter of the transistor62. This causes a negative pulse to be applied to the transistor 60 ofthe next stage of the current driver.

The transistor 60 has its base electrically connected to the collectorof the transistor 62 through a ohm, /2 watt resistor 74 and to thesource 66 through a 2K, /2 watt resistor 76. The emitter of thetransistor 60 is grounded and its collector is connected to one end ofthe series connection of reset windings, to the anode of the 1N400 9diode 78, and to one end of the 2.5 millihenry, 9.3 ohm inductor 80. Theother end of the inductor 80 is connected to the source 70 through a 96ohm resistor 82. The cathode of the diode 7 8 is electrically connectedto the anode of the 1N4009 diode 84. The cathode of the diode 84 isconnected to the end of the set line 52 opposite to the seriesconnection of the set windings of the cores 54. A ohm resistor 86 iselectrically connected at one end to the source 70 and at the other endto the end of the reset line 58 opposite the collector of the transistor60.

Whenever a negative pulse is received on the base of the transistor 60from the first stage of the current driver in response to input triggerpulses, the normally conducting transistor 60 is cut off. The potentialof the collector of the transistor 60 is approximately 0.5 volt when itis conducting and it is approximately 40 volts when it is first cut oif.This 40 volt swing is the result of a transient generated by theinductor 80 when the opening of the transistor switch 60 tends tointerrupt the flow of current through the inductor. This voltage swingcauses a high current to fiow through the two diodes 78 and 84 andthrough the set windings of the thirteen cores 54 setting these cores.Approximately three volts per core are required.

When the transistor '60 is conducting, as it is between input pulses tothe terminal 56, current flows through the reset line 58 from the source70 to ground. This current resets the cores 54. Setting current does notflow through the set line 52 because it is blocked by the two diodes 78and 84. These diodes each have 0.7 of an ohm forward resistance at 2milliamperes. They may be thought of as self-actuated switches whichpresent a relatively high resistance until they are forward biased,after which they present a low forward resistance. Of course, thetransistor 60 is a separately-actuated switch since it is triggered byan input to its base from the previous stage of the current driver.

It can be seen that the embodiment of the invention shown in FIG. 3 hasall of the advantages of the circuit shown in FIG. 1 plus an additionaladvantage: it is capable of both setting and resetting cores. It has lowpower dissipation and uses relatively inexpensive mediumpowertransistors.

In FIG. 4, a graph is shown of four curves having common abscissae oftime and individual ordinates of current. The curve 88 represents thecurrent through the inductor 80. It can be seen that this current isrelatively constant even though the circuit to which it flows changes:including, first, a path to ground through the transistor 60, and then,a path to ground through the set windings of the cores 54.

The curve 90 represents the current through the set line 52. Thiscurrent is negligible for 64-rnicrosecond intervals indicated by theregion 92 and has a pulse amplitude equal to the DC inductance currentfor 4-microsecond intervals such as that indicated by 94. Theseintervals correspond to the time-width of the input pulses applied tothe input terminal 56.

The third curve 96 represents current through the reset line 58. Thiscurrent is relatively high for 64-millisecond intervals such as that inthe region 98 and falls to a negligible value for 4-microsecondintervals such as those indicated at 100. The fourth curve 102represents the current through the transistor 60. It can be seen thatthis current is normally high during 6 4-microsecond intervals such asthose indicated at 104 and falls to negligible amount during4-microsecond intervals such as those indicated at 106. It is theseinterruptions that cause the voltage transients at the collector of thetransistor 60 to open the two diode switches 78 and 84 to current flowfrom the inductor '80.

It is noted that the sum of the currents indicated by the curves 90 and102 is a constant equal to the current through the inductor 80 andrepresented by the curve 88.

In FIG. 5, a schematic circuit diagram of another modification of theinvention is shown. This modification of the invention has the sameoutput circuit as the circuit of FIG. 3, including the source ofpotential 70, the resistor 82, the resistor 86, the reset line 58, thecores 54, the diode 84, the diode 78, the set line 52, the inductor 80,and the transistor 60. However, the input circuit has been modified tosave one stage of the logical circuitry used to apply trigger pulses tothe current driver and also to speed up the operation of the currentdriver. In this modification of the invention it is possible to activatethe current driver by applying negative pulses to the input terminal 56.Also, the transistor 108 in the first stage of the current driver isbiased to operate in the Class A mode so that it is never saturated.This speeds up its operation by reducing minority carrier storage. Sincethe input circuit stage is an emitter-follower type, it has highercurrent gain which can be advantageously used to saturate the outputtransistor.

The base of the NP-N transistor 108 is electrically connected to theinput terminal 56 through a resistor 110 and to a source of a negative 8volts 112 through the resistor 114. The emitter of the transistor 108 isdirectly connected to the base of the transistor 60 and also connectedto the source 112 through the resistor 116. The value of the resistor116 in this circuit is relatively high compared to the value of thecorresponding resistor 76 in the circuit of FIG. 3. The collector of thetransistor 108 is connected to the source 70 through a resistor 118.

It can be seen that the first stage of this current driver is not aninverter. When it receives a negative going input pulse on terminal 56,it conducts a negative pulse to the base of transistor 60 cutting thistransistor off and switching the setting current to the setting windingsof the cores 54. The transistor 108 does not saturate but merelyoperates as an amplifier to conduct these pulses to the transistor 60.

It can be seen that the current drivers of this invention are simple,economical, and reliable. Also, they do not dissipate an excessiveamount of power. Because of the lower power requirements, less expensivecomponents can be used. Also, many components serve two functions, thatof setting and that of resetting the cores, resulting in further economyand reducing errors from noise. If a core is set by noise, it isautomatically reset so that it is ready for use when the next set pulseis applied.

Of course, many modifications and variations of the invention arepossible in the light of the above teachings. It is therefore to beunderstood, that within the scope of the appended claims, the inventionmay be practiced otherwise than as specifically described.

What is claimed is:

1. A current driver adapted to provide current pulses to a load througha first terminal and a steady flow of current to a load through a secondterminal, comprising:

an inductor adapted to be connected at one end to a source of current;

gating means having a predetermined voltage threshold electricallyconnected between the second end of said inductor and the firstterminal;

control means electrically connected to the second end of said inductorto alternately charge said inductor and discharge said inductor toprovide high current pulses to the first terminal; and

current-limiting means connected to said one end of said inductor tolimit the steady flow of current to the second terminal to an amountsmaller than that flowing into said inductor.

2. A current driver for setting ferromagnetic cores comprising:

a set conductor adapted to be connected to the set windings of saidferromagnetic cores;

a reset conductor adapted to be connected to the reset windings of saidferromagnetic cores;

a first NPN transistor having its emitter grounded and its collectorelectrically connected to one end of said reset conductor;

a first diode having its anode electrically connected to the collectorof said first NPN transistor;

a second diode having its anode electrically connected to the cathode ofsaid first diode and having its cathode electrically connected to saidset conductor;

said reset windings of said ferromagnetic cores being adapted to beconnected to a source of positive potential at the opposite end fromtheir connection to said reset conductor;

said set windings being adapted to be grounded at the opposite end astheir connection to said set conductor;

an inductor having an inductance greater than the inductance of said setwindings and being electrically connected at one end to the collector ofsaid NPN transistor and at its other end being adapted to be connectedto a source of electrical potential;

a second NPN transistor having its emitter grounded, and having itscollector electrically connected to the base of said first NPNtransistor;

an input terminal adapted to receive positive input triggering pulsesand being electrically connected to the base of said second NPNtransistor;

first biasing means for biasing said first NPN transistor to saturation;and

second biasing means for biasing said second NPN transistor to cut off.

3. A current driver for ferromagnetic cores comprising:

a plurality of set windings each wound around a different one of saidferromagnetic cores;

said plurality of set windings being connected in series from aset-winding input terminal to a set-winding output terminal;

a plurality of reset windings each wound around a different one of saidferromagnetic cores;

said plurality of reset windings being connected in series from areset-winding input terminal to a reset winding output terminal;

a first NPN transistor having its emitter grounded and having itscollector electrically connected to said reset windings input terminal;

a first diode having its anode electrically connected to the collectorof said first NPN transistor;

a second diode having its anode electrically connected to the cathode ofsaid first diode and having its cathode electrically connected to saidset winding input terminal;

said reset winding output terminal being adapted to be connected to asource of positive potential;

said set winding output terminal being grounded;

an inductor having one end electrically connected to the collector ofsaid first NPN transistor and having its other end adapted to beconnected to said source of positive potential;

first bias means for biasing said first NPN transistor so as to benormally conducting;

a second NPN transistor having its emitter electrically connected to thebase of said first NPN transistor; second biasing means for biasing saidsecond NPN transistor so as to be normally conducting; and

an input terminal adapted to receive negative-going input pulses andbeing electrically connected to the base of said second NPN transistor,whereby said second NPN transistor is cut off by said input pulses so asto provide a negative pulse to the base of said first NPN transistor,cutting it off also.

4. A ferromagnetic core actuator circuit comprising:

an inductor connected at one end to a source of current,

control means electrically connected to said inductor controlling theflow of charging current in said inductor in a normal position,

a load comprising at least one ferromagnetic core having a set windingand a reset Winding, said reset winding connected in electrical parallelcircuit with said inductor and in electrical series circuit with saidcontrol means to reset said ferromagnetic core in the normal position,and

gating means electrically connecting said set winding in electricalparallel circuit with said control means and operable to set saidferromagnetic core by discharging said inducto-r through said setwinding when said control means interrupts the flow of charging currentthrough said inductor.

5. The ferromagnetic core actuating circuit according to claim 4 whereinthe inductance of said inductor is greater than the inductance of saidload.

6. The ferromagnetic core actuating circuit according to claim 4 whereinthe control means includes a transistor normally biased in a state ofconduction.

7. The ferromagnetic core actuating circuit according to claim 4 whereinthe gating means has a predetermined voltage drop independent of theamount of current flowing through said element and said voltage drop isgreater than the voltage drop across said control means in the normalposition.

References Cited UNITED STATES PATENTS 3,065,358 11/1962 Lee et a1.307-88 BERNARD KONICK, Primary Examiner.

G. M. HOFFMAN, Assistant Examiner.

U.S. Cl. X.R. 307-238, 270

